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Nov 03, 2020, 06:37 AM
Melbourne, Australia
Thread OP
Mini-HowTo

Quick and Dirty CF frame construction


Hi Folks,

... it's been a bit crazy with coronavirus around here, and I haven't had much time for airship building, but I thought I'd do some experimenting with carbon fibre frame construction, suitable for a small rigid model. Some previous models of mine on this list have made heavy use of 3D printed parts, but these are not ideal - they are relatively heavy, and very brittle. So I ordered a supply of 2m lengths of 1mm round carbon fibre 'rod', and had a bit of a play with different glues - some of my results are probably well known to folks already, but some may be of interest.

Firstly, I reproduced some of the results others have had with glue; I tried a number of glues on lightly sanded carbon fibre that was cleaned with isopropyl alcohol:

cyanoacrylate: quick setting, very brittle, breaks easily
two part epoxy ('araldite'): slow setting, quite strong, will snap under load
UV quick setting glue: convenient, sets in seconds under UV, not super strong, 'good enough' for many applications.
'gorilla' modelling glue: slow setting, didn't seem to bond well with the CF

... however my personal 'breakthrough' was trying standard woodworking 'expanding polyurethane' glue (sold as Selley's Aquabond here in Aus, but I think there's a whole range of brands selling it, including UHU and hobbyists). I found this was brilliant for my purposes, due to its high flexibility - it allowed the frame to flex significantly, particularly when I smeared it all the way around both rods, creating a 'wrapped' bond.

Based on this I tossed together a very quick frame, based on some hoops (ends joined with - *cough* - wrapped dental floss - and then glued with epoxy), following which the frame was quick to construct using the polyurethane glue.

One 'novel' feature was I held the frame in position with paperclips, which worked very well at keeping the joints together while the PU glue dried. I did it in stages, although I think I could have pretty much done it in one shot if I'd wanted to.

Some piccies attached; it's really just a test piece to look at how practical a small rigid model might be - it's about 91cm long and 30cm wide, and ended up weighing in at ~13.7g. (Theoretical lift ~35g for the shape, but this isn't intended as a practical model, rather it's a test unit.)
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Nov 04, 2020, 02:10 PM
Go small or go home
ruzam's Avatar
Good to see you're back in the 'lab' developing new things!

Tell me more about this 'expanding polyurethane'? It sounds a lot like what is sold here as 'white' Gorilla Glue. Unlike regular Gorilla Glue it's mostly clear out of the bottle. It sets in 10-15 minutes and foams up white (much more if you pre-mix a little water with it). As a model working glue I find it to be about as good as it gets, used sparingly. Just enough working time to get things set up, but quick enough set that you can sit and hold it if you have to.
Nov 24, 2020, 12:28 PM
Registered User
Wow, pretty cool construction there! I see that you managed to build some very small radius frames. How far would these rods bend?
Jul 17, 2021, 12:05 PM
Potential Future Has-Been
3D printed joints don't have to be heavy and brittle, you just need to use the right filament. Standard PLA is too brittle, but eSun makes a PLA+ version that is pretty tough and works well. The ultimate filament is the carbon fiber filled nylon, particularly the Oynx brand used with Mark Forged printers. It's pretty expensive at $200 per Kg though, and then you need the $3000 Mark Forged printer to really make the best possible joints. I have been involved with some very large CF rigid models built using 3D printed joints however and they work great. Wish I could post pics, there are some epic rigid models in existence now.
Jul 17, 2021, 11:52 PM
Melbourne, Australia
Thread OP
Hi Pyro!

*waves*

Good to hear that there's high quality CF printing available now. I had a look a few years ago and folks were selling PLA with short embedded fibres that apparently didn't actually add any strength - it was all a bit dodgy. Nice to know there are good printers now, sounds like just the thing for an all-CF build! I wonder if anyone is printing good quality CF as a service; the actual parts volume is pretty small, so it might not be too expensive...

I was actually tempted by the current generation of resin printers; they look like they've got great resolution, and don't have the 'weak direction' problem of the deposition printers... however apparently they smell like crazy and you need to spend a lot of time washing the parts with exciting chemicals, both of which could lead to some negative feedback from some local stakeholders...

Hmm... looks like I missed a few messages here, sorry!

@Ruzam - yes indeed, gorilla glue looks like the same stuff - I'm using a local equivalent (Sikens tech grip) - but the key is to smear it fully around both rods, creating a 'rubber band' like effect holding them together, which keeps things flexible. I want to say 'tensegrity mesh' but I probably shouldn't...

@rentz - I did some experiments on this; I think the 1mm had a minimum diameter of 12cm, but there was some variation between visually identical rods. In practice I never go under 15cm though, because it's too brittle and likely to break... sometimes days later...
Jul 18, 2021, 12:45 PM
Potential Future Has-Been
We did make some SLA joints using a FormLabs resin printer several years ago back when that was the only consumer SLA printer on the market. There is a lot more SLA printers available now, but we got away from resin prints due to how brittle they are and, as you mentioned, the chemicals involved. We trashed the floor of an entire room due to resin drips and such. MarkForged is really the ultimate thing to use for CF joint nodes if you can afford it. This isn't the printers that lay continuous fiber, which would be unworkable for such tiny joints and also overkill. It is still chopped fiber embedded in nylon, which is nowhere near as strong as a continuous fiber type construction by certainly strong enough for making CF frame joints. The resolution of the MarkForged printers is unmatched too, you can barely see the layer lines. They can only print that one type of filament though, so it is pretty inflexible as an all around printer. A Prusa i3 printing eSun PLA+ would probably be good enough, and then you would have a printer that can run all types of filament and much more affordable.


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